A plurality of fins comprising thin sheets are provided on outer surfaces of tubes which are members constituting a heat exchanger such as a radiator for a motorcycle or an automobile, a condenser for a cooler, or an evaporator, in order to increase the radiating surface area of the tube. An aluminum alloy is used in many cases as the material for the aforementioned tubes, and in such a case, it is the usual practice to use, as the fin material, thin sheets of an aluminum alloy, or clad sheets manufactured by applying a brazing metal film to one or both sides of these sheets as core materials.
Various methods are available for installing fins of any of the aforementioned materials on the outer surface of a tube made of an aluminum alloy. Among such various methods, the vacuum brazing method for brazing many joints at a time is widely adopted in the industry, which comprises: provisionally tacking a plurality of fins substantially vertically at prescribed intervals onto the outer surface of a tube by the use of an appropriate jig to form an assembly, at least one of the outer surface of said tube and the both surfaces or one surface of said fin being previously applied with a brazing metal film; heating said assembly to a temperature of from about 580.degree. to about 620.degree. C. in a vacuum furnace to melt said brazing metal film, thereby brazing said plurality of fins to the outer surface of said tube with said melted brazing metal at a time by a single heating.
When installing fins onto the outer surface of a tube by the aforementioned method, heating causes a change in the structure of fin comprising aluminum alloy sheets or clad sheets, leading to finer recrystallization grains, and when the fins comprise clad sheets, the above-mentioned heating causes penetration of the melted brazing metal between fine recrystallization grains, resulting in a decrease in the high-temperature strength of the fins; the fins sag down and deform as a result and cannot hold their original shape upon forming the assembly.
To overcome the above-mentioned problems, an Al--Mn alloy or an Al--Mn--Zr alloy has so far been employed as the material for aluminum alloy sheets constituting fins. Manganese contained in the Al--Mn alloy and zirconium contained in the Al--Mn--Zr alloy prevent recrystallization grains from becoming finer during heating, and in the case of fins comprising clad sheets, the aforementioned prevention of recrystallization grains from becoming finer makes it difficult for the melted brazing metal to penetrate between recrystallization grains, thus providing the effect of increasing high-temperature strength of the fins.
However, high-temperature strength of the fins is still insufficient even with the above-mentioned Al--Mn alloy or Al--Mn--Zr alloy. Particularly, as a result of the recent tendency toward decreasing the thickness of fins to reduce the weight of a heat exchanger, it is difficult to prevent fins from sagging down and deforming under the effect of the aforementioned heating for brazing.
With this fact in view, there has been proposed an aluminum alloy excellent in high-temperature sagging resistance, adapted to be used as a fin material for tubes of a heat exchanger, as disclosed in Japanese Patent Publication No. 9,617/82 dated Feb. 22, 1982, which comprises:
manganese: from 0.6 to 1.5 wt.%, PA1 magnesium: from 0.3 to 0.9 wt.%, PA1 silicon: from 0.05 to 0.25 wt.%, PA1 zirconium: from 0.05 to 0.25 wt.%, and PA1 vanadium: from 0.01 to 0.1 wt.%, PA1 manganese: from 0.1 to 0.5 wt.%, PA1 silicon: from 0.1 to 0.8 wt.%, PA1 zirconium: from 0.02 to 0.2 wt.%, PA1 magnesium: from 0.1 to 0.7 wt.%, and, PA1 copper: from 0.1 to 0.7 wt.%, PA1 manganese: from 0.1 to 0.5 wt.%, PA1 silicon: from 0.1 to 0.8 wt.%, PA1 zirconium: from 0.02 to 0.2 wt.%, PA1 magnesium: from 0.1 to 0.7 wt.%, and, PA1 copper: from 0.1 to 0.7 wt.%,
at least one element selected from the group consisting of:
and, the balance being aluminum and incidental impurities (hereinafter referred to as the "prior art").
In the above-mentioned prior art, however, manganese contained in an amount of at least 0.6 wt.% in the aluminum alloy leads to the presence of a large amount of manganese dissolved in aluminum, and the resultant decreased thermal conductivity causes deterioration of heat exchange property as a fin material for a heat exchanger. In the prior art, furthermore, when the disclosed aluminum alloy is used as a fin material for tubes of a heat exchanger, the high manganese content in the aluminum alloy makes the fin electric potential noble, resulting in acceleration of corrosion of the tube, although the fins must play the role of a sacrificial anode to cathodically protect the tube from corrosion.